It is known from practice to detect and monitor the state of a lubricant of a bearing, in particular of a roller bearing or journal bearing, and specifically the chemical composition of the lubricant. Changes in the chemical state of the lubricant provide indications of aging processes in the lubricant and indicate when the lubricant must be replaced or replenished.
Within the scope of this application, a “lubricant” is any lubricating substance which is used for lubrication in a bearing, reduces friction and wear and, in some cases, can also be used to carry out further functions, for example for power transmission between the bearing components, for cooling of the bearing, as corrosion protection, for vibration damping, or else as a sealant.
It is known for capacitance measurements to be carried out on the lubricant. However, in this case, layer thicknesses have a major influence on the measurement result. Furthermore, it has been found that the measurement results are widely scattered and are difficult to reproduce. In addition, capacitance measurements provide only an indirect indication of the chemical composition of the lubricant, for example, the measurement results of capacitance measurements are influenced by metallically conductive particles located in the lubricant.
It is also known for a sample of the lubricant to be taken from the bearing and to be irradiated by means of electromagnetic radiation, in order to record and to analyze a spectrum of the sample. Particularly known is the IR-spectroscopic analysis of lubricant grease samples outside the bearing, in the spectral range of the middle and/or near infrared (MIR and NIR). The measurement is carried out outside the bearing, that is to say not subject to the specific physical or chemical constraints in the bearing interior. Furthermore, it is complex to take samples, particularly if access to the bearing is difficult and/or the bearing must be shut down for the sample to be taken. Furthermore, the location within the bearing at which the sample has been taken has an influence on the measurement result of the spectroscopy, since the lubricant is a mixture of components which are distributed within the bearing interior during operation of the bearing. Another problem that has been found is that the lubricant ages to a different extent at different locations within the bearing.
DE 35 10 408 A1 describes an apparatus for monitoring the operating state of bearings. In this case, the instantaneous state of the lubricant of the bearing is determined by providing a trapping device in the immediate vicinity of the bearing, which traps and analyzes lubricant emerging from the bearing. The analysis is restricted to verification of metallic particles which are contained in the lubricant of the bearing, and to the detection of the temperature of the lubricant. No provision is made for any statement relating to the chemical composition of the lubricant in the interior of the bearing.
DE 93 11 938 U1 describes an apparatus for taking a sample of lubricant, in particular lubricating grease, from a roller bearing. In this case, bores are introduced into the bearing rings of the roller bearing, through each of which lubricating grease samples can be taken. In this case as well, immediate in-situ measurement of the state of the lubricant is impossible.